1. Field of the Invention
This invention relates to subsea operations. More particularly, method and apparatus are provided for applying force to move a conductor or other member into a selected location for joining with another member, such as a conductor which is to be joined to a subsea wellhead.
2. Description of Related Art
Wells drilled in the water-covered areas of the earth have become an important source of energy in recent decades. There has been a steady increase in the number of wells in deeper water, and many types of apparatus and procedures are used for drilling and producing these wells. Progressing from drilling and producing wells that are serviceable by divers at moderate cost, in water depths less than about 500 feet, to wells that are in water depths of several thousand feet, it has been necessary to develop many new procedures and types of apparatus.
One of the procedures used in drilling of subsea wells in deeper water is to drill the wells from a mobile drilling rig--which may be either jack-up or floating. After the wells are drilled and completed on the sea floor, a fixed platform is constructed above the wells. The wells are drilled in a small surface area on the sea floor, through a template which contains the number of "slots", or openings which is at least as great and the number of wells. Extra slots for contingent wells normally are present. After the wells are drilled, each well is fitted with a subsea wellhead before the mobile drilling rig is moved off the wells.
To construct a platform above the wellheads, a survey is made from the subsea template on the sea floor to fixed locations in which piles are to be driven to support or anchor the platform. The platform may be fixed on legs or may be buoyant with various types of anchoring means to the sea floor. Fixed platforms with stationary legs are often the most economical form of platform down to water depths in the range of about 1,000 feet, but for greater water depths some form of buoyant or compliant platform structure is often used.
After the platform is constructed, a large casing string, often called the "environmental conductor," must be run from the platform at the sea surface to the wellhead of each well drilled. The wellhead is then "tied back" to the platform by the use of the environmental conductor. This may be 20-inch diameter casing, a 26-inch diameter casing or other large diameter casing. Water depths are often greater than divers normally operate or, at least, diving expenses are very high in water depths used. Therefore, it is common to use remotely operated vehicles (ROVs) to perform operations at the sea floor. ROVs are widely used in the offshore industry, and normally have capabilities for lighting and television viewing and have mechanical arms for manipulation of apparatus. The platforms normally contain guides for the environmental conductors, which are attached to the support members of the platform. The guides are normally placed about 100 or 150 feet apart down the platform structure and over each wellhead. The guides have an inside diameter from about two inches to about 20 inches larger than the environmental conductor which is to be employed to tie back the wellheads. Centralizers may be placed on the environmental conductor such that they will position that pipe to enter the wellhead when the guides are at least several inches larger than the environmental conductor. Centralizers may be formed to be eccentric such that when they are present in guides nearest the wellhead they allow the environmental conductor string to enter the wellhead upon lowering of the string into place. In other platform developments, centralizers are not used and the guides in the platform are sized so as to position the environmental conductor over the wellhead so that it can enter the wellhead upon lowering if the platform is properly placed.
Since the environmental conductor is confined to guides, for the conductor to enter the subsea wellhead the platform must be precisely located over the wellheads. If the platform has not been placed within one or two inches of its desired location, it may not be possible to lower the environmental conductor into its wellhead for tieback. If divers can be used, it is common to pull the environmental conductor over the wellhead by applying a winch between legs of the platform and the environmental conductor and applying force to pull the bottom of the environmental conductor over the wellhead. Below diver depth, however, where ROVs are used for sea floor operations, a winch is not easily used and there is no method available to apply sufficient force to place the environmental conductor over the wellhead so that it can be attached. The force supplied by available ROVs is inadequate.
The use of water jets to apply force for propelling boats is known. For example, U.S. Pat. No. 4,461,620 discloses a water intake and conduit for conveying water from the intake to an aperture. U.S. Pat. No. 3,937,172 discloses a water jet propelling apparatus including jet-deflecting surfaces for controlling the thrust or deviating the jet. The use of high-pressure jets for displacing equipment used in subsea wells is not known to the inventor.
There is a need for method and apparatus which may used to position an environmental conductor string of casing over a subsea wellhead or to apply force to other subsea equipment. The method should be operable with the aid of ROVs or other means of manipulating the apparatus and viewing its position at or near the sea floor. The method and apparatus should be adaptable to a wide range of water depths and operating conditions, should employ some of the apparatus which is readily available on an offshore platform and should be simple and inexpensive to operate in the environment of use.